The invention described herein was made in the course of, or under, Energy Research and Development Administration Contract No. W-7405-ENG-48 with the University of California. The work was supported in part by the Environmental Protection Agency and the National Science Foundation.
This invention relates to the removal of pollutants from effluent gases from stationary sources. More particularly, the invention relates to the simultaneous removal of SO.sub.x and NO.sub.x from effluent gases of fossil fuel burning power plants.
Emissions of nitric oxide and sulfur dioxide from stationary sources have been an increasing problem in the United States over the last several years. Assuming no controls, it is estimated that by the year 1980, power plants would emit about 65% of the total sulfur dioxide emissions and about 25% of total NO emissions. Although both of these pollutants are generated from the same source, separate control technology has heretofore been developed for each.
In December, 1971, the Environmental Protection Agency promulgated Standards of Performance for New Stationary Sources which included limits on the emissions of both SO.sub.x and NO.sub.x from power plants. This clearly identified the need to develop processes which controlled both SO.sub.2 and NO emissions. A feasible process for the simultaneous removal of nitrogen oxides and sulfur oxides from the effluent gases of fossil-fuel burning power plants should meet the following criteria: (1) the process should have the potential of lowering the NO.sub.x and SO.sub.x concentrations to less than 100 parts per million; (2) no other type of pollution problem should be created by installation of the process; (3) the removal mechanism for one component should not depend upon the presence or absence of the other; (4) furnace operation should be independent of the process operation; (5) the process should be adaptable to both new and existing power plants, and (6) the economics should be an improvement over the best currently known processes.
The simultaneous reduction of nitric oxide and sulfur dioxide by carbon monoxide in the presence of a copper catalyst supported on alumina is known. (See C. W. Quinlan, V. C. Okay, J. R. Kittrell, "Simultaneous Catalytic Reduction of Nitric Oxide and Sulfur Dioxide by Carbon Monoxide", Ind. Eng. Chem. Process Des. Develop. 12 (3) 359, 1973.) In the aforementioned process elemental sulfur is formed by the direct reduction of the sulfur dioxide; however, the reaction of carbon monoxide and sulfur also produces carbonyl sulfide, a toxic and undesirable side product. The production of carbonyl sulfide is a major difficulty with this method. In order to obviate this problem, it has been proposed to utilize three catalyst beds operating at two different temperatures (see R. Querido, W. L. Short, "Removal of Sulfur Dioxide from Stack Gases by Catalytic Reduction to Elemental Sulfur with Carbon Monoxide", Ind. Eng. Chem. Process Des. Develop. 12 (1) 10, 1973). According to this proposal, the entire gas stream would pass through a first catalyst bed of copper oxide on alumina at about 485.degree. C to remove oxygen, which poisons the catalyst, by conversion to Co.sub.2. The exit stream would then be split, the major portion entering the second reactor containing Cu on alumina catalyst, also at 485.degree. C. In the second reactor, SO.sub.2 would be reduced to sulfur and COS. The effluent from the second reactor, along with the smaller split flow from the first reactor, would then be cooled to about 315.degree. C passed into a third reactor also containing a Cu on alumina catalyst bed. In the third reactor, the COS formed in the second reactor would react with SO.sub.2 to form sulfur.
The above-described prior art process, while it accomplishes the simultaneous removal of nitric oxide and sulfur dioxide, leaves much to be desired. First, three catalyst beds in series will create a large system pressure drop. Second, the bypass stream must be very accurately controlled to provide stoichiometric COS and SO.sub.2. Third, the amount of CO required for the reduction must be closely controlled since only reactions with O.sub.2, SO.sub.2 and NO remove it. Fourth, the flue gas would have to be cooled and then reheated after the bed to insure complete precipitation of the sulfur. Fifth, failure in the performance of the first bed would result in O.sub.2 poisoning the remaining two.